The photographic arts blog.

How Filters Change Tonal Separation in B&W Photos

I posted a B/W (monochrome) shot on Flickr a few weeks back and mentioned that I’d used a particular B&W color filter effect to get it right. Later, it occurred to me that not everybody might know what I meant by that; how it works. So, while I’m no big expert filters, I kind of like explaining stuff… and that was the tipping point which motivated starting this blog. Here, then is what I wanted to mention about filters:

This is the photo I put up on Flickr.

The sun illuminated the scene very strongly from the left, while the sky overhead was intense dark-blue (dawn blue), casting the scene in shade. Now notice that I said “casting … in shade”. Most of us think of shade as the absence of sunlight, forgetting that something is reflecting light onto the scene otherwise it would be inky blank. That reflected light is what we call “shade”. And shade has color. Actually, it has lots of colors from all the many things reflecting – casting – their light where the sun don’t shine. Shade is both complex and diffuse; that’s where the action is.

In contrast, sunlight comes from a point light source and is unvarying in its spectral makeup; along with being intense, it is consistent and concentrated. In the realm of B&W photography, sunlight is very, very bright and very white; everything else down to jet black is diffused and some shade of gray. Because the shade of gray you see is based on it’s color, colored filters can be used to alter the relative brightnesses among objects or areas in a scene.

This is the original color exposure, straight out of the camera. It was beautiful, but to me, color really didn’t capture the mood – what I felt – standing there. Then it hit me; color was actually obscuring the the quality of the light!

The overall contrast is very high, but large areas are very soft; in the back, where the grey pilings meet the yellowish tan grass, the brightness of the grasses and pilings are quite similar, but the colors are different, and color differences can be converted to brightness differences with filters.

The Color Wheel

The color wheel is your friend. It unlocks the “secret” of filters, which is this:

A filter of a color on one side of the wheel reduces the apparent brightness of the color on the other side.

Get that? Reduces the color on the other side.

Take a look at that wheel on the right.

There’s a little ‘pip’ on the right where it’s red, marking this as being a “red” filter. Now, notice that right across from red on the other side of the color wheel is what? Sky Blue (cyan actually). So, in B&W photography, what does a red filter do? It reduces the brightness of the sky colors. It can turn the sky black (as in Ansel Adams’ famous shot of Half Dome).

So, look around the edge of the color wheel at the various colors and imagine those colors in nature. You see; blue sky, green foliage, orange sunsets, skin tones, etc. You can adjust the relative brightness of these in a B&W photo by using a filter of the color on the opposite side of the wheel.

That’s pretty much all you need to know

Examples:

When thinking about filter colors, in the field on on a computer screen in processing, you have to start by considering the color – the visual color, the actual color of the things in the scene. As I’m sure you know, something which is pure white in bright sunlight appears orange-ish in a room lit by incandescent bulbs. That’s why we set the white balance in camera; so that a “white” thing appears “white” when the image is viewed. The color you see is the color of the light source, less the colors the object absorbs (rather than reflecting). So, different color source, different color seen.

Outside in daylight there are two primary light sources (neglecting things like fill flash): sunlight, and sunlight reflected off of something else. The something elses absorb some of the colors from the sun and reflect the rest. Hence “blue sky” (aka shade) is a common second light source.

Here’s a blown up insert from the shot above. What do you see? The bright foreground highlights on the grasses are illuminated by the sun; the parts in shade are illuminated largely by the sky. The colors in the shade are mostly pale yellows (the grasses), greens (foliage and younger grass, and dark chocolate browns. But the pilings have a distinct bluish cast.

NB: the filters shots below often appear quite grainy; this can be used for effect, or easily cleaned up.

No Filter

Note the pip is dead center in the color wheel – no color filter. This is the B&W direct conversion of the color shot. Note that the separation of the pilings from the background is minimal, just as it appears in the color image.

Red Filter

A red filter reduces levels of cyan, so now the slightly bluish pilings stand out well from the grasses. Also, the grasses in the front are brighter. More dramatically, the bright grasses at the upper left – almost invisible in the color shot, are now much more distinct.

Orange Filter

An orange filter darkens “azure” blue; the pilings are darker still, and the chocolate shaded foreground is actually a bit lighter – a bit more detail has been revealed.

Yellow Filter

Good grief! The yellow filter removes BLUE, period. So a strong Yellow filter like this turns things predominantly blue BLACK. Note the background in the shrubs above the grasses; textures are revealed.

Chartreuse Filter

Chartreuse? Yeah…. Violet is reduced – some blue has come back, but anything green in the color shot is deepening in tone. Note particularly the greenish patch in the bright clump of grasses in the lower left quadrant; its shadowed area is rather detailed now.

Green Filter

Anything magenta goes to black; separation of the pilings from the grasses remains high. Qualitatively, compare this with the red filter; the foreground is now much darker and the overall contrast is much higher.

Aqua Filter

Aqua removes rose-colored light – more red than blue. So the pilings (bluish) are darkened, but the grasses (reddish) are darkened even more. Overall this is nice, but those bright grasses in the upper left quadrant are more subdued now. is that what you’d want (decisions, decisions…)?

Cyan Filter

Say goodbye to red… the pilings have jumped in brightness and now blend with the background grasses again; but the texture in the shrubbery at the back has increased. Anything blue or bluish is now accentuated.

Azure Filter

Azure removes orange. Look at the background grasses; the contrast in the foliage is enhanced far beyond what the color or B&W shot reveal. The ground is now heavily textured.

Blue Filter

One of the classics, a blue filter reduces the yellow components of the scene; the background grasses are now textured heavily, while the shadowed area of the bright clump of grass in the lower left foreground is nearly black.

Violet Filter

Violet reduces chartreuse. Look back at the color image; there is a substantial chartreuse – “limey green” – component in much of the shot. The matted grassy areas between the pilings and the background tend to dominate the scene.

Magenta Filter

Magenta reduces green. Anything green has darkened as much as possible – what’s left are the reds a blues. As a result, the separation between the pilings and grass in the back are gone.

Rose Filter

Rose reduces Aqua. Notice in particular how the contrasts in the foliage in the back have dropped dramatically compared to the magenta filter, just 30 degrees away on the color wheel. The effect is softer than the red filter and is subtly different from “no” filter, but with dramatically better separation of pilings in the rear.

Summary

As you can see, filters give you enormous control over the relative brightnesses in a scene. Results vary from subtle to dramatic. Which to use? Depends on your intent. More subtle effects heighten realism – accentuate what was seen and/or felt while remaining true to “reality”. Other filters can be used to explore the hidden world – the relationships we never see in “reality”; shade can become more dramatic than the brightly lit portions of the original scene.

NOTE:The color wheel shots are screen captures from Lightzone, a photo processing tool I use frequently. You can grab and drag that little “pip” on the color wheel to any part of the wheel and see the results instantly; download the free trial and try this out on your own photos.